other hand, are not as well understood. If silver is transported as a sulfide complex,
boiling should also cause silver to precipitate. However, if silver is transported as a
chloride complex boiling is less likely to result in silver precipitation. Recent
experimental studies by Pokrovski et al. (2008) at 350 – 500°C (temperatures somewhat
higher than those in the epithermal environment) showed that the solubility of silver in
chloride solutions decreases with increasing pH, reflecting a change from transport as a
chloride complex in acidic solutions to transport as S-complexes in neutral to basic
solutions. Thus, the changes in solution properties described above as a result of boiling,
specifically an increase in pH, will also promote silver deposition if silver is transported
as a sulfur complex.
In this study, the relationship between fluid inclusion and mineral textural
evidence for boiling and precious metal grades in the classic epithermal Ag-Au deposits
at Guanajuato, Mexico, have been investigated to test for systematic correlations that may
be used in exploration for similar deposits. Approximately 850 samples were collected
from surface outcrops, underground workings, and recent drill core over a strike length of
4 km and to depths of 750 m beneath the surface in four active mines and one closed
mine along the Veta Madre. Each sample was assayed for Au, Ag, Cu, Pb, Zn, As, Sb.
The goal of this study was to develop a method to evaluate the economic potential
samples based only on data obtained during petrographic examination of thin sections
and does not require time-consuming and expensive microthermometry or microanalysis
of the inclusions.